1. Field of the Invention
The present invention relates to a sheet feeding apparatus wherein stacked sheets are separated by separating claws and are fed one by one.
2. Related Background Art
In recording systems such as printers, copying machines and the like, or in original reading systems, a cut sheet feeder has been used to separate a sheet (including an original such as a printed matter, as well as a recording medium such as a paper) from a sheet stack one by one to be fed successively.
As disclosed in the U.S. Pat. Nos. 3,531,106, 4,4492,371 and 4,653,743, various types of sheet separating mechanisms have been proposed, one of which utilizes separating claws. The present invention is directed to a sheet feeding apparatus including such separating claws.
FIG. 26 shows a plan view of a conventional separating claw and FIG. 27 shows an elevational view of the separating claw of FIG. 26.
Referring to FIGS. 26 and 27, in a sheet feeding apparatus, a number of sheets (plain sheets) 11 are stacked on pressure plates 12 and the sheet stack is pressed against sheet supply rollers 14 by biasing the pressure plates 12 upwardly by means of bias springs 13. Separating claws 15 are arranged at both corners of the front end of the sheet stack 11, by which the uppermost sheet 11 is separated from the sheet stack one by one when the frictional feeding force of the supply rollers 14 is created.
The separating claws 15 and the supply rollers 14 are paired, respectively, and are arranged symmetrically with respect to a sheet feeding direction, respectively. Only one (left side one) of these elements 15, 14 are shown in FIGS. 26 and 27.
Each separating claw 15 has a pawl portion 16 for holding down an upper surface of the sheet stack 11 and an abutment portion 17 against which the front end of the sheet stack 11 is abutted. The separating claws are constituted by discrete members each other and are arranged at left and right sides of a front end of a frame of the sheet feeding apparatus integrally therewith or pivotably for up-and-down movement.
FIGS. 28A, 28B and 28C show schematically, in sectional view, three stages of the sheet separating and feeding method performed by the separating claws with respect to the sheets 11.
In FIG. 28A, the sheets 11 are in a condition that the sheets stacked on the pressure plates 12 are pressed against the supply rollers 14 by the bias springs 13. In this condition, the supply rollers 14 are rotated in the direction shown by the arrow in response to a sheet feed command signal. When the supply rollers 14 are rotated, a thrust force due to the friction force is applied on the sheet stack 11.
On the other hand, at the both sides of the front end of the sheet stack, the upper surface of the sheet stack is pressed down by the pawl portions 16 and the front end of the sheet stack is abutted against the abutment portion 17. Accordingly, as shown in FIG. 28B, the uppermost sheet on the sheet stack 11 is bent to form a loop between the pawl portions 16 and the supply rollers 14.
When the supply rollers 14 are further rotated until a predetermined amount of the loop is obtained, the uppermost sheet 11 will ride over the pawl portions 16 due to the elastic restoring force of the sheet itself, so that it is fed toward a sheet feeding direction as shown in FIG. 28C, thus separating the uppermost sheet from the sheet stack 11. The separated sheet 11 is fed by the supply rollers 14 until it is abutted against feed rollers provided in a recording portion of a recording system, and thereafter, the sheet is fed to a predetermined position in the recording portion, thus preparing for a recording operation and the like.
However, in the conventional claw separating mechanism, for example, if thicker sheet having high rigidity such as a postcard, drawing paper and the like is used, the loop (as shown in FIG. 28B) cannot be created in the sheet since the feeding force (friction force) of the supply rollers cannot overcome the hardness to be bent (of the sheet), with the result that the sheet cannot ride over the separating pawl portions 16.
Consequently, in the conventional sheet feeding apparatus, the automatic feeding of both the normal sheet (such as the plain paper) and the thicker sheet (such as a postcard) could not be attained, unless both a sheet feeding apparatus for the normal sheet and a sheet feeding apparatus for the thicker sheet are provided or unless a thicker sheet holder including a thicker sheet separating mechanism is arranged in the normal sheet feeding apparatus.